Spatial risk assessment on land subsidence in Beijing, China

Abstract

Land subsidence is the lowering of land-surface elevation mainly due to human activities including groundwater withdrawal, oil or gas pumping. It is a critical threat to the sustainable development of urban constructions and social economy. Long-term and large-scale over-exploitation of groundwater, which supplies about two-thirds of the total regional water requirement, is a main cause for land subsidence in Beijing Municipality, China. It is essential to assess land subsidence risks for decision-makers to prevent the disaster. This study takes the up-middle part of alluvial-proluvial plain fan of the Chaobai River in Beijing as an example area, where Huairou Emergency Groundwater Resource Region is located. It evaluates the risk of land subsidence between 2006 and 2008 by adopting the analytical hierarchy process with sensitivity analysis (AHP-SA) method. Six criteria used for the evaluation are thicknesses of compressible sediment and the quaternary strata, changes in groundwater level of the unconfined and confined aquifer system, building density and recharge from precipitation infiltration. Criteria weights were determined on the basis of the interrelation coefficients between land subsidence and six factors. The highest weight was assigned to thickness of compressible sediment, followed by the thickness of quaternary strata and change in confined aquifer groundwater level. The weight of recharge from precipitation infiltration is the lowest. Distribution of land subsidence during the same period, which was derived by using Persistent Scatter Synthetic Aperture Radar Interferometry technique, was used to verify the accuracy of the risk assessment map. The land subsidence risk map shows that the high risk region covers about 150 km 2 and is mainly distributed in the southwest of the study area. Area of moderate risk accounts for nearly 1/3 of the whole area which is in the middle-southern area. No-hazard zone is located in the northern area, which is the upper area of the alluvial-pluvial plain fan. The evaluation results are least sensitive to the recharge from precipitation infiltration. Risk map is more sensitive to the change of confined aquifer groundwater level in that when the weight changes for more than ± 2%, an obvious shift between high and moderate subsidence risk regions occurs. The risk map is also sensitive to the thicknesses of the quaternary and compressible sediment. However, since these two factors cannot be controlled by human beings, it is necessary for decision-makers to pay more attention to limit the drawdown of groundwater level. The output of this study provides a better insight to land subsidence hazard management.